1. Field of the Invention:
The present invention relates to a wiring board on which an electronic component is mounted by a flip chip bonding system and to a method for producing such a wiring board.
2. Description of the Related Art:
In recent years, the size of semiconductor devices has been reduced more, and accordingly, the number of terminals of a semiconductor device has remarkably been increased. In mounting a semiconductor chip on a wiring board, therefore, it has become difficult to microscopically connect terminals of the semiconductor chip to the wiring board by conventional wire bonding. Today, the wire bonding system is replaced with a flip chip bonding system.
FIG. 5 is a cross sectional view of a wiring board 17 on which a semiconductor chip 10 is mounted by use of a flip chip bonding system. The semiconductor chip 10 having bumps 12 is mounted on the wiring board 17 by face down bonding. The bumps 12 are formed of solder, gold or the like and are provided on pads 11 on the semiconductor chip 10. The wiring board 17 includes an electrically insulating plate 13 and a specified wiring pattern 14 provided thereon. The semiconductor chip 10 is connected to the wiring pattern 14. The semiconductor chip 10 and the wiring board 17 are fixed and the connection therebetween is secured by an adhesive layer 15 provided therebetween and formed of a resin or the like. Such a conventional flip chip bonding system is described in "Handbook for Semiconductor Mounting" (published on Sep. 25, 1986; pp. 128-138).
In FIG. 5, portions of a surface of the wiring pattern 14 act as bonding pads 18, to which the bumps 12 of the semiconductor chip 10 are connected by soldering. The bonding pads 18 and the vicinity thereof are formed of a metal which can easily be soldered. In order to prevent solder from flowing out through the bonding pads 18, a solder dam structure is employed, by which the wiring board 17 including the wiring pattern 14 is prevented from being touched with solder except for the bonding pads 18. The solder dam structure is constructed by forming an insulating film such as a solder resist 16 on a surface of the insulating plate 13 having the wiring pattern 14 thereon, except for the bonding pads 18, as is shown by a two-dot chain line of FIG. 5. The solder resist 16 is generally patterned by use of a printing process or a photographic process. Thus, the above-mentioned flowing-out of solder are prevented.
FIG. 6a is a plan view of the wiring board 17 having the above-mentioned conventional solder dam structure. The solder resist 16 is formed on the insulating plate 13 having the wiring pattern 14 thereon except for the bonding pads 18. Each bonding pad 18 has a width W3 of approximately 100 .mu.m. FIG. 6b is a plan view of the solder resist 16, which has substantially circular holes 19 at positions corresponding to the bonding pads 18. Such a solder dam structure having substantially circular holes is described in "Okidenki Research and Development, 138, Vol. 55, No. 2" (published in April 1988; pp. 51-56).
The above solder dam structure has a problem in that it is extremely difficult to form the microscopic bonding pads 18 with high precision by use of a printing process. For example, if the pattern shown in FIG. 6b is formed with a positional deviation of approximately 100 .mu.m, the holes 19 are made at positions 40 between wirings of the wiring pattern 14, or at positions 50 having a distance from leading edges of the bonding pads 18 as is shown in FIG. 7. With such a wiring board, the connection between the semiconductor chip 10 and the wiring pattern 14 is possibly defective or broken, resulting in a low reliability in terms of connection.
When a photographic process is used for patterning the solder resist 16, the positional deviation of the holes 19 is avoided due to the highly precise positioning of the photographic process. However, formation of the solder dam structure requires a complicated production procedure and a long period of time, thus increasing the production cost of the wiring board.